3 May 2019
If you cast a card "without paying its mana cost," you can't choose to cast it for any alternative costs. You can, however, pay additional costs. If the card has any mandatory additional costs, such as that of Spark Harvest, you must pay those to cast the card.
3 May 2019
If a spell has {X} in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.
3 May 2019
As Finale of Promise resolves, first you cast the target instant card and/or the target sorcery card in either order. Then, if X is 10 or more, you copy each of those twice and put the copies on the stack in any order. The copies will resolve before the original spells.
3 May 2019
You can't choose to pay any additional costs for the copies. However, effects based on any additional costs that were paid for the original spell are copied as though those same costs were paid for the copy too.
3 May 2019
If the spell that's copied is modal (that is, it says "Choose one—" or the like), the copies will have the same mode or modes. You can't choose different ones.
3 May 2019
A split card that's an instant and a sorcery card may be the target of Finale of Promise twice. However, once you cast that card once, it can't be cast a second time.
3 May 2019
The copies that Finale of Promise creates are created on the stack, so they're not "cast." Abilities that trigger when a player casts a spell won't trigger.
3 May 2019
If the spell has damage divided as it was cast, the division can't be changed (although the targets receiving that damage still can). The same is true of spells that distribute counters.